Abstract
Complexation by transition metal ions (Cu^II^ and Fe^II^) was successfully used to differentiate the diastereomeric YAGFL, Y^D^AGFL and Y^D^AGF^D^L pentapeptides by electrospray ionization‐ion trap mass spectrometry in the positive ion mode using low‐energy collision conditions. This distinction was allowed by the stereochemical effects due to the ^(D)^Leu/^(L)^Leu and the ^(D)^Ala/^(L)^Ala residues yielding various steric interactions which direct relative dissociation rate constants of the binary [(M − H) + Me^II^]^+^ complexes (Me = Cu or Fe) subjected to low‐energy, collision‐induced dissociation processes. The interpretation of the collision‐induced dissociation spectra obtained from the diastereomeric cationized peptides allowed the location of the deprotonated site(s), leading to the postulation of ion structures and fragmentation pathways for both the [(M − H) + Cu^II^]^+^ and [(M − H) + Fe^II^]^+^ complexes, which differed significantly. With Cu^II^, consecutive fragmentations, initiated by the decarboxylation at C‐terminus, were favored relative to sequence product ions. On the other hand, with Fe^II^, competitive fragmentations resulting in abundant sequence product ions and significant internal losses were preferred. This could be explained by different localizations of the negative charge, which directs the orientation of both the [(M − H) + Cu^II^]^+^ and [(M − H) + Fe^II^]^+^ binary complexes fragmentations. Indeed, the free negative charge of the [(M − H) + Cu^II^]^+^ ions was mainly located at one oxygen atom: either at the C‐terminal carboxylic group or, to a minor extent, at the Tyr phenol group (i.e. zwitterionic forms). On the other hand, the negative charge of the [(M − H) + Fe^II^]^+^ ions was mainly located at one of the nitrogen atoms of the peptide backbone and coordinated to Fe^II^ (i.e. salt non‐zwitterionic form).
Moreover, this study reveals the particular behavior of Cu^II^ reduced to Cu^I^, which promotes radical losses not observed from the peptide–Fe^II^ complexes. Finally, this study shows the analytical potentialities of the complexation of transition metal ions with peptides providing structural information complementary to that obtained from low‐energy, collision‐induced dissociation processes of protonated or deprotonated peptides. Copyright © 2006 John Wiley & Sons, Ltd.